Nowadays, deep oil fields to which consideration has never been given, sour gas fields whose development was abandoned due to their harsh corrosive environments, and so forth are being actively developed on a global scale from the viewpoint of a sharp rise in the price of crude oil and the exhaustion of oil resources which is anticipated in the near future. Such oil fields and gas fields are generally found very deep in the ground and in harsh corrosive environments in which the atmosphere has a high temperature and contains CO2, Cl−, and so forth. Therefore, as a steel pipe for oil country tubular goods which are used in order to drill such oil fields and gas fields, there is a strong demand for a seamless steel pipe having not only a high strength corresponding to a yield stress of higher than 654 MPa (95 ksi), but also excellent corrosion resistance.
In response to such requirements, for example, Patent Literature 1 describes martensitic stainless steel excellent in terms of corrosion resistance and sulfide stress cracking resistance. The martensitic stainless steel according to Patent Literature 1 is martensitic stainless steel excellent in terms of corrosion resistance and sulfide stress cracking resistance having a chemical composition containing, by weight %, C: 0.005% to 0.05%, Si: 0.05% to 0.5%, Mn: 0.1% to 1.0%, P: 0.025% or less, S: 0.015% or less, Cr: 10% to 15%, Ni: 4.0% to 9.0%, Cu: 0.5% to 3%, Mo: 1.0% to 3.0%, Al: 0.005% to 0.2%, and N: 0.005% to 0.1%, in which the condition that Ni equivalent is −10 or more is satisfied, and a microstructure including a tempered martensite phase, a martensite phase, and a retained austenite phase, in which the sum of the fraction of a tempered martensite phase and the fraction of a martensite phase is 60% to 90%, and Patent Literature 1 states that it is possible to manufacture the martensitic stainless steel by performing a two-step heat treatment including a heat treatment at a temperature equal to or lower than a temperature at which an austenite phase fraction is 80% and a heat treatment at a temperature equal to or lower than a temperature at which an austenite phase fraction is 60%. Patent Literature 1 states that, with this, it is possible to achieve excellent hot workability, a yield stress of 551 MPa to 861 MPa (80 ksi to 110 ksi), improved corrosion resistance in an aqueous carbon dioxide environment, and improved sulfide stress cracking resistance in an aqueous hydrogen sulfide environment.
In addition, Patent Literature 2 describes a stainless steel pipe for oil country tubular goods. The steel pipe according to Patent Literature 2 is a stainless steel pipe for oil country tubular goods excellent in terms of corrosion resistance having a chemical composition containing, by mass %, C: 0.05% or less, Si: 0.50% or less, Mn: 0.20% to 1.80%, Cr: 14.0% to 18.0%, Ni: 5.0% to 8.0%, Mo: 1.5% to 3.5%, Cu: 0.5% to 3.5%, Al: 0.05% or less, V: 0.20% or less, N: 0.01% to 0.15%, and O: 0.006% or less, in which the relationships Cr+0.65Ni+0.6Mo+0.55Cu−20C≥18.5 and Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≤11 are satisfied, and Patent Literature 2 states that it is possible to obtain the steel pipe by performing, after a pipe-making process has been performed, a quenching treatment including performing heating to a temperature equal to or higher than the Ac3 transformation temperature and then performing cooling to room temperature at a cooling rate equal to or larger than that of air cooling and performing a tempering treatment at a temperature equal to or lower than the Ac1 transformation temperature. Patent Literature 2 states that, with this, it is possible to achieve a high strength corresponding to a yield stress of higher than 654 MPa (95 ksi) and excellent corrosion resistance even in a harsh corrosive environment containing CO2, Cl−, and so forth and having a temperature higher than 180° C. and equal to or lower than 230° C.
In addition, Patent Literature 3 describes a high-strength stainless steel pipe for oil country tubular goods excellent in terms of corrosion resistance. The steel pipe according to Patent Literature 3 is a steel pipe having a chemical composition containing, by mass %, C: 0.005% to 0.05%, Si: 0.05% to 0.5%, Mn: 0.2% to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5% to 18%, Ni: 1.5% to 5%, Mo: 1% to 3.5%, V: 0.02% to 0.2%, N: 0.01% to 0.15%, and O: 0.006% or less, in which the relationships Cr+0.65Ni+0.6Mo+0.55Cu−20C≥19.5 and Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≥11.5 are satisfied, and, preferably, a microstructure including a martensite phase as a base phase, a ferrite phase in an amount of 10% to 60% or more in terms of volume fraction, and, optionally, a retained austenite phase in an amount of 30% or less in terms of volume fraction. Patent Literature 3 states that, with this, it is possible to achieve improved hot workability so that cracking in a pipe-making process is prevented, a high strength corresponding to a yield stress of higher than 654 MPa (95 ksi) and excellent corrosion resistance even in an harsh corrosive environment which contains CO2, Cl−, and so forth and which has a temperature of 230° C.